Periodontal diseases are among the most common infectious diseases and constitute a major cause of tooth loss in adults. The ability of periodontal pathogens, such as the gram-negative species Fusobacterium nucleatum and Porphyromonas gingivalis, to adhere to other plaque microorangisms is though to be essential in colonization, and thus a key event in the infectious process. F. nucleatum T18 adheres to P. gingivalis T22 and the major outer membrane protein, Apg (Adherence to Porphyromonas Gingivalis), has been implicated as an adhesion in this interaction. The hypothesis to be tested in this investigation is that Apg functions as an adhesion in the adherence of F. nucleatum T18 to P. gingivalis T22. To examine this hypothesis, the first question to be addressed is: Do adherence-defective (A-) mutants of F. nucleatum result from a mutation in apg? We propose to construct an apg- mutant of F. nucleatum T18 using shuttle mutagenesis. The apg- mutant will be characterized, including examination of the expression of Apg and adherence properties. Insertional inactivation of a gene is a powerful tool to identify the function of the gene product, but it is important to rule out the possibility of polar effects resulting from the gene disruption. Thus the second question to be addressed is: Does complementation of an adherence-defective (A-) apg- mutant with the will type apg locus result in reversion to the adherence- effective (A+) phenotype? Complementation studies will be performed using the A-, apg- mutant and a plasmid vector that will be developed F. nucleatum. These studies will enhance our understanding of the molecular basis of F. nucleatum - P. gingivalis adherence, and provide the tools essential for further studies addressing the colonization and pathogenesis of both F. nucleatum and P. gingivalis. Additionally, this research involves the development of systems of manipulation that will be essential for the study of the molecular basis of any phenotypic property of F. nucleatum and possibly other related gram- negative anaerobic pathogens.